Problem: 10 glue sticks cost $19.70. Which equation would help determine the cost of 7 glue sticks?
Explanation: There are several equations that could help determine the cost, each with a slightly different approach. We know the cost of 10 glue sticks. We want to know the cost of 7 glue sticks. We can write the numbers of glue sticks as a proportion: $\dfrac{10}{7}$ We know 10 glue sticks costs $19.70. We can let $x$ represent the unknown cost of 7 glue sticks. The proportion of these costs can be expressed as: $\dfrac{\$19.70}{x}$ The cost changes along with the number of glue sticks purchased, and so the two proportions are equivalent. $\dfrac{10}{7} = \dfrac{\$19.70}{x}$